It is known that transition metal sulphides make it possible to perform hydrogenation reactions in the presence of large amounts of sulphur-containing molecules. Indeed, contrary to the standard hydrogenation catalysts, such as metals, the sulphide phases are not poisoned by sulphur-containing molecules. Their use for the catalytic reduction of aromatic disulphides to mercaptans is already described in the literature. Thus:
Patent Application NL 6,402,424 claims the use of metal sulphides, in particular platinum sulphide, for reducing diphenyl disulphide to phenyl mercaptan; PA1 for this same reaction, Patents FR 2,008,331 and DE 1,903,968 recommend the metals Raney Ni or Raney Co, as well as the metals Ru, Rh, Pt, Ir and Pd or their sulphides; PA1 according to Patent Application JP 56-81541, cobalt sulphide allows the reduction of aromatic compounds of the type RS.sub.n R' to mercaptans RSH and
R'SH, R and R' being phenyl, p-nitrophenyl or 3,4-dichlorophenyl radicals.
If these processes are performed generally at relatively mild reaction temperatures (approx. 200.degree. C.), the hydrogen pressures are, on the contrary, very high (50-100 bar) and catalyst/reactant ratios of the order of 2 are employed. In addition, the reaction is carried out in a closed reactor and in a three-phase medium: gas-liquid-solid.
The selective catalytic reduction of dimethyl disulphide to methyl mercaptan does not appear to have already been described. On the other hand, the total hydrogenolysis of dimethyl disulphide is widely used for the sulphuration of the hydrotreatment catalysts, which has become the most important industrial application of DMDS; in this case, DMDS is a hydrogen sulphide precursor which is the sulphuration agent of these catalysts. Work relating to the sulphuration of hydrotreatment catalysts using DMDS demonstrates that, during the presulphuration period of Co--Mo and of Ni--Mo catalysts, the distribution of the DMDS decomposition products (methyl mercaptan, dimethyl sulphide, hydrogen sulphide and methane) changes as a function of the temperature, a low temperature (of around 200.degree. C.) favoring the formation of methyl mercaptan and that of dimethyl disulphide. On the other hand, at a higher temperature, hydrogenolysis of methyl mercaptan is rapid and leads to the predominant formation of hydrogen sulphide and methane.
Control of the hydrogenation of dimethyl disulphide in order to produce methyl mercaptan exclusively according to the reaction: EQU CH.sub.3 --SS--CH.sub.3 +H.sub.2 .fwdarw.2CH.sub.3 --SH
proves to be very difficult, given that these two compounds possess C-S bonds which are sensitive to hydrogenolysis, readily leading to the formation of hydrogen sulphide and methane. Furthermore, the additional formation of dimethyl sulphide, observed with catalysts based on transition metals (Co--Mo and Ni--Mo), is a factor which penalizes the production of methyl mercaptan.
Very large excesses of hydrogen, with respect to dimethyl disulphide, are used for the sulphuration of the hydrotreatment catalysts. By working with high proportions of hydrogen, the partial hydrogenation of DMDS limited to methyl mercaptan is disadvantaged with respect to a more complete hydrogenolysis of the sulphur-containing organic compounds.
However, by working with much lower hydrogen/DES ratios, side-reactions may occur on the catalyst and in particular favor formation of dimethyl sulphide by the reaction: EQU 2 CH.sub.3 SH.fwdarw.CH.sub.3 SCH.sub.3 +H.sub.2 S